The present invention generally relates to head servo circuits in magnetic reproducing apparatuses, and more particularly to a head servo circuit designed so that a head servo system for controlling the rotational speed of rotary heads locks in instantaneously or locks in within an exceedingly short period of time when a mode of a magnetic reproducing apparatus is switched over to normal reproduction mode from high-speed reproduction mode.
In a magnetic recording and reproducing apparatus which records and reproduces a video signal onto and from tracks formed obliquely on a magnetic tape with respect to the longitudinal direction o the magnetic tape, recording positions of a vertical synchronizing signal of the video signal must be aligned at end parts of the oblique and parallel recording tracks upon recording. On the other hand, upon reproduction, rotary heads must accurately scan over the above recording tracks. Hence, in this type of a so-called helical scan type magnetic recording and reproducing apparatus (hereinafter simply referred to as a magnetic recording and reproducing apparatus), a head servo circuit is generally provided. This head servo circuit comprises a speed control loop for controlling the rotational speed of the rotary heads, and a phase control loop for controllingg the rotational phase of the rotary heads.
In the above magnetic recording and reproducing apparatus, high-speed search reproduction can be performed to search for a predetermined recording position. During the high-speed search reproduction, the magnetic tape is cauded to travel in the same direction as upon recording (forward direction) or in a reverse direction at a high speed on the order of ten times the tape traveling speed upon normal reproduction, for example. Because the magnetic tape travels at a high speed in the forward or reverse direction during the high-speed search reproduction as described above, the relative scanning linear speed between the rotary heads and the magnetic tape during the high-speed search reproduction differs with that upon normal reproduction. Accordingly, the horizontal scanning frequency of the reproduced video signal upon high-speed search reproduction differs from the regular horizontal scanning frequency upon normal reproduction. Thus, if the reproduced signal obtained during the high-speed search reproduction is supplied to a television receiver, the reproduced picture will be out of horizontal synchronism, and a regular reproduced picture cannot be obtained.
Therefore, in order to obtain a reproduced picture in horizontal synchronism even during the high-speed search reproduction, it becomes necessary to carry out reproduction so that the horizontal scanning frequency of the reproduced video signal becomes the same as the horizontal scanning frequency upon normal reproduction (15.734 kHz when the video signal is an NTSC system color video signal, for example). In order to achieve this, it becomes necessary to rotate the rotary heads at an increased or decreased speed compared to the speed upon normal reproduction according to the direction of the tape travel, during the high-speed search reproduction. That is, if the tape traveling direction during the high-speed search reproduction is the same as the tape traveling direction (forward direction) upon normal reproduction, the rotary heads must be rotated at a rotational speed higher than that upon normal reproduction. On the other hand, if the tape traveling direction is in the reverse direction during the high-speed search reproduction, the rotary heads must be rotated at a rotational speed lower than that upon normal reproduction. Hence, in the above head servo circuit, the phase control loop is cut off during the high-speed search reproduction, and moreover, a voltage obtained subjecting the horizontal scanning frequency of the horizontal synchronizing signal to frequency-to-voltage conversion is applied to a motor (hereinafter referred to as a head motor) for rotating the rotary heads. Hence, the rotational speed of the rotary heads is controlled so that the relative scanning linear speed between the rotary heads and the magnetic tape is the same as that upon normal reproduction during the high-speed search reproduction.
However, in the phase control loop of the above head servo circuit, the frequency and phase of a reference signal used for obtaining a phase error signal are the same during the normal reproduction and the high-speed search reproduction. On the other hand, the frequency and phase of a phase comparison signal detected according to the rotation of the rotary heads and compared with the above reference signal in order to obtain the phase error signal, differ during the normal reproduction and the high-speed search reproduction. As a result, in the conventional head servo circuit, when the mode of the magnetic recording and reproducing apparatus is switched over to the normal reproduction mode from the high-speed search reproduction mode, the relationship between the frequency and phase of the phase error signal and the frequency and phase of the reference signal becomes unstable, and the phase control loop does not lock-in instantaneously. Thus, there was a disadvantage in that it takes a time on the order of one to several seconds for the phase control loop to lock-in from the point in time when the mode is switched over, and regular reproduction of the picture could not be carried out during the time it takes for the phase control loop to lock-in.